Cooled concrete played a big part in producing a nearly crack-free base mat for a Milwaukee high-rise building. The 6-foot-thick reinforced concrete mat required almost 8,000 cubic yards of cooled concrete. Crack control was critical because of the planned usage, a 90,000-square-foot computer center entirely below grade. Designers carefully planned waterproofing details to eliminate any possibility of downtime caused by water leakage. Reducing the number of cracks in the base mat to near zero was part of the overall waterproofing plan.
REDUCING TEMPERATURE DIFFERENCES WITHIN THE MAT
Heat doesn't escape rapidly from the middle of a 6-foot-thick concrete mat. As the cement hydrates, released heat builds up and interior concrete expands. But at the surface, where heat is lost faster, the concrete doesn't expand as fast. It may even contract. The differences in volume changes caused by differences in temperature create stresses that can crack the concrete. To control the temperature differences, design engineers used two strategies: reduce heat generation by keeping the cement content low; and hold temperature differences within the mat to less than 30 degrees Fahrenheit. A 60 degree Fahrenheit limit on delivered concrete helped keep the temperature at the center of the mat no more than 30 degrees above the temperature at the surface. The concrete contractor kept concrete cool by injecting liquid nitrogen at the two ready mix plants supplying concrete for the job.